Modeling a bipyramidal shape
Create the bi-pyramid's base. 

If the base is going to be a regular polygon, just do: Add > Circle, then look for this menu to change the number of vertices. The menu is obtained using F6 in Blender &lt;2.79, or you can just find it in the bottom-left of the Viewport in Blender2.8+


Jump into Edit Mode (Tab). Select all the vertices (A), then Extrude upwards in the Z direction (E then Z). If you hold Ctrl after starting the extrusion, you can do it by snapping to fixed increments.


Deselect the top vertices, select the bottom vertices (I used the B Box Select tool), and Extrude them downwards


For both the top and bottom ends: select them, then scale to zero: S, 0, Enter


To finish, select all the vertices (A), then do Remove Double Vertices from the right mouse button menu (Blender 2.8+) or from the W button menu (&lt;2.79)

The mesh is already done after step 4, but step 5 gets rid of redundant geometry and open surfaces (which is a requirement for many operations, including boolean, subsurface, 3D printing).